Melanotan II Research Articles

Antidepressant-like and antistress effects of the ACTH(4–10) synthetic analogs Semax and Melanotan II on male rats in a model of chronic unpredictable stress

Current antidepressant therapy shows substantial limitations, and there is an urgent need for the development of new treatment strategies for depression. Stressful events and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis play an important role in the pathogenesis of depression. HPA axis activity is self-regulated by negative feedback at several levels including adrenocorticotropic hormone (ACTH)-mediated feedback. Here, we investigated whether noncorticotropic synthetic analogs of the ACTH(4–10) fragment, ACTH(4–7)-Pro-Gly-Pro (Semax) and Ac-Nle4-cyclo[Asp5-His6-D-Phe7-Arg8-Trp9-Lys10]ACTH(4–10)-NH2 (Melanotan II (MTII), a potent agonist of melanocortin receptors), have potential antidepressant activity in a chronic unpredictable stress (CUS) rat model of depression. Stressed and control male adult Sprague-Dawley rats received daily intraperitoneal injections of saline or a low dose (60 nmol/kg of body weight (BW)) of Semax or MTII. Rats were monitored for BW and hedonic status, as measured in the sucrose preference test. We found that chronic treatment with Semax and MTII reversed or substantially attenuated CUS-induced anhedonia, BW gain suppression, adrenal hypertrophy and a decrease in the hippocampal levels of BDNF. In the forced swim test, no effects of the CUS procedure or peptides on the duration of rat immobility were detected. Our findings show that in the CUS paradigm, systemically administered ACTH(4–10) analogs Semax and MTII exert antidepressant-like effects on anhedonia and hippocampal BDNF levels, and attenuate markers of chronic stress load, at least in male rats. The results support the argument that ACTH(4–10) analogs and other noncorticotropic melanocortins may have promising therapeutic potential for the treatment and prevention of depression and other stress-related pathologies.

Recommended Tool Compounds for the Melanocortin Receptor (MCR) G Protein-Coupled Receptors (GPCRs).

The melanocortin receptors are a centrally and peripherally expressed family of Class A GPCRs with physiological roles, including pigmentation, steroidogenesis, energy homeostasis, and others yet to be fully characterized. There are five melanocortin receptor subtypes that, apart from the melanocortin-2 receptor (MC2R), are stimulated by a shared set of endogenous agonists. Until 2020, X-ray crystallographic and cryo-electron microscopic (cryo-EM) structures of these receptors were unavailable, and the investigation of their mechanisms of action and putative ligand-receptor interactions was driven by site-directed mutagenesis studies of the receptors and targeted structure-activity relationship (SAR) studies of the endogenous and derivative synthetic ligands. Synthetic derivatives of the endogenous agonist ligand α-MSH have evolved into a suite of powerful ligands such as NDP-MSH (melanotan I), melanotan II (MTII), and SHU9119. This suite of tool compounds now enables the study of the melanocortin receptors and serves as scaffolds for FDA-approved drugs, means of validating stably expressing melanocortin receptor cell lines, core ligands in assessing cryo-EM structures of active and inactive receptor complexes, and essential references for high-throughput discovery and mechanism of action studies. Herein, we review the history and significance of a finite set of these essential tool compounds and discuss how they are being utilized to further the field's understanding of melanocortin receptor physiology and greater druggability.

Multiple metabolic signals in the CeA regulate feeding: The role of AMPK

BackgroundThe central nucleus of the amygdala (CeA) is part of the dopaminergic reward system and controls energy balance. Recently, a cluster of neurons was identified as responsive to the orexigenic effect of ghrelin and fasting. However, the signaling pathway by which ghrelin and fasting induce feeding is unknown. AMP-activated protein kinase (AMPK) is a cellular energy sensor, and its Thr172 phosphorylation (AMPKThr172) in the mediobasal hypothalamus regulates food intake. However, whether the expression and activation of AMPK in CeA could be one of the intracellular signaling activated in response to ghrelin and fasting eliciting food intake is unknown. AimTo evaluate the activation of AMPK into CeA in response to ghrelin, fasting, and 2-deoxy-D-glucose (2DG) and whether feeding accompanied these changes. In addition, to investigate whether the inhibition of AMPK into CeA could decrease food intake. MethodsOn a chow diet, eight-week-old Wistar male rats were stereotaxically implanted with a cannula in the CeA to inject several modulators of AMPKα1/2Thr172 phosphorylation, and we performed physiological and molecular assays. Key findingsFasting increased, and refeeding reduced AMPKThr172 in the CeA. Intra-CeA glucose injection decreased feeding, whereas injection of 2DG, a glucoprivation inductor, in the CeA, increased food intake and blood glucose, despite faint increases in AMPKThr172. Intra-CeA ghrelin injection increased food intake and AMPKThr172. To further confirm the role of AMPK in the CeA, chronic injection of Melanotan II (MTII) in CeA reduced body mass and food intake over seven days together with a slight decrease in AMPKThr172. SignificanceOur findings identified that AMPK might be part of the signaling machinery in the CeA, which responds to nutrients and hormones contributing to feeding control. The results can contribute to understanding the pathophysiological mechanisms of altered feeding behavior/consumption, such as binge eating of caloric-dense, palatable food.

Melanocortin agonism in a social context selectively activates nucleus accumbens in an oxytocin-dependent manner

Social deficits are debilitating features of many psychiatric disorders, including autism. While time-intensive behavioral therapy is moderately effective, there are no pharmacological interventions for social deficits in autism. Many studies have attempted to treat social deficits using the neuropeptide oxytocin for its powerful neuromodulatory abilities and influence on social behaviors and cognition. However, clinical trials utilizing supplementation paradigms in which exogenous oxytocin is chronically administered independent of context have failed. An alternative treatment paradigm suggests pharmacologically activating the endogenous oxytocin system during behavioral therapy to enhance the efficacy of therapy by facilitating social learning. To this end, melanocortin receptor agonists like Melanotan II (MTII), which induces central oxytocin release and accelerates formation of partner preference, a form of social learning, in prairie voles, are promising pharmacological tools. To model pharmacological activation of the endogenous oxytocin system during behavioral therapy, we administered MTII prior to social interactions between male and female voles. We assessed its effect on oxytocin-dependent activity in brain regions subserving social learning using Fos expression as a proxy for neuronal activation. In non-social contexts, MTII only activated hypothalamic paraventricular nucleus, a primary site of oxytocin synthesis. However, during social interactions, MTII selectively increased oxytocin-dependent activation of nucleus accumbens, a site critical for social learning. These results suggest a mechanism for the MTII-induced acceleration of partner preference formation observed in previous studies. Moreover, they are consistent with the hypothesis that pharmacologically activating the endogenous oxytocin system with a melanocortin agonist during behavioral therapy has potential to facilitate social learning.

Melanotan-II reverses memory impairment induced by a short-term HF diet

A high-fat (HF) diet has been shown to increase the risk of neurological impairments and neurodegenerative disorders. The melanotropins used in this study have been associated with diet-related disorders; however, there is an absence of studies on their effect on diet-induced neurobehavioral conditions. Here, we investigated the possible relationship among diet, Melanotan-II (MT-II) targeting melanotropin receptors, and the behavior of zebrafish (Danio rerio). Surprisingly, even a short-term HF diet lasting for ∼ 1 % of the zebrafish's life had a strong developmental effect. Zebrafish fed the HF diet showed an impairment in recognition memory, elevated anxiety levels, and reduced exploratory propensity after just three weeks compared to zebrafish fed the control diet. These HF diet-induced abnormalities were reversed by MT-II. Animals fed a HF diet and treated with MT-II demonstrated recognition memory, anxiety, and exploratory behavior similar to the control group. This study provides evidence that even a short-term HF diet has an impact on memory and emotions and is the first study to show that MT-II reverses these changes.

CLIPSing Melanotan-II to Discover Multiple Functionally Selective hMCR Agonists.

The pleiotropic role played by melanocortin receptors (MCRs) in both physiological and pathological processes has stimulated medicinal chemists to develop synthetic agonists/antagonists with improved potency and selectivity. Here, by deploying the Chemical Linkage of Peptide onto Scaffolds strategy, we replaced the lactam cyclization of melanotan II (MT-II), a potent and unselective agonist of human MCRs (hMCRs), with different xylene-derived thioethers. The newly designed peptides displayed binding affinities toward MCRs ranging from the low nanomolar to the sub-micromolar range, highlighting a correlation between the explored linkers and the affinity toward hMCRs. In contrast to the parent peptide (MT-II), compound 5 displayed a remarkable functional selectivity toward the hMC1R. Enhanced sampling molecular dynamics simulations were found to be instrumental in outlining how the employed cyclization strategy affects the peptides' conformational behavior and, as a consequence, the detected hMC1R affinity. Additionally, a model of the peptide 5/hMC1R complex employing the very recently reported cryogenic electron microscopy receptor structure was provided.

Zebrafish Bioassay for Screening Therapeutic Candidates Based on Melanotrophic Activity.

In this study, we used the zebrafish animal model to establish a bioassay by which physiological efficacy differential of alpha-melanocyte-stimulating hormone (α-MSH) analogues could be measured by melanosome dispersion in zebrafish larvae. Brain-skin connection research has purported the interconnectedness between the nervous system and skin physiology. Accordingly, the neuropeptide α-MSH is a key regulator in several physiological processes, such as skin pigmentation in fish. In mammals, α-MSH has been found to regulate motivated behavior, appetite, and emotion, including stimulation of satiety and anxiety. Several clinical and animal model studies of autism spectrum disorder (ASD) have already demonstrated the effectiveness of α-MSH in restoring the social deficits of autism. Therefore, we sought to analyze the effect of synthetic and naturally-occurring α-MSH variants amongst different species. Our results showed that unique α-MSH derivatives from several fish species produced differential effects on the degree of melanophore dispersion. Using α-MSH human form as a standard, we could identify derivatives that induced greater physiological effects; particularly, the synthetic analogue melanotan-II (MT-II) exhibited a higher capacity for melanophore dispersion than human α-MSH. This was consistent with previous findings in an ASD mouse model demonstrating the effectiveness of MT-II in improving ASD behavioral symptoms. Thus, the melanophore assay may serve as a useful screening tool for therapeutic candidates for novel drug discovery.

The Mesencephalic Trigeminal Nucleus Controls Food Intake and Body Weight via Hindbrain POMC Projections.

The mesencephalic trigeminal nucleus (Mes5) processes oral sensory–motor information, but its role in the control of energy balance remains unexplored. Here, using fluorescent in situ hybridization, we show that the Mes5 expresses the melanocortin-4 receptor. Consistent with MC4R activation in other areas of the brain, we found that Mes5 microinjection of the MC4R agonist melanotan-II (MTII) suppresses food intake and body weight in the mouse. Furthermore, NTS POMC-projecting neurons to the Mes5 can be chemogenetically activated to drive a suppression in food intake. Taken together, these findings highlight the Mes5 as a novel target of melanocortinergic control of food intake and body weight regulation, although elucidating the endogenous role of this circuit requires future study. While we observed the sufficiency of Mes5 MC4Rs for food intake and body weight suppression, these receptors do not appear to be necessary for food intake or body weight control. Collectively, the data presented here support the functional relevance of the NTS POMC to Mes5 projection pathway as a novel circuit that can be targeted to modulate food intake and body weight.

Melanotan II, a melanocortin agonist, partially rescues the impaired thermogenic capacity of pituitary adenylate cyclase-activating polypeptide deficient mice.

What is the central question of this study? Can chronic treatment of pituitary adenylate cyclase-activating polypeptide (PACAP) deficient mice with the melanocortin agonist melanotan II during cold acclimation rescue the impaired thermogenic capacity previously observed in PACAP deficient mice? What is the main finding and its importance? Using a genetic model of PACAP deficiency, this study provides evidence that PACAP acts upstream of the melanocortin system in regulating sympathetic nerve activity to brown adipose tissue in mice. Impaired adipose tissue function in obesity, including reduced thermogenic potential, has detrimental consequences for metabolic health. Hormonal regulation of adaptive thermogenesis is being explored as a potential therapeutic target for human obesity. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide expressed in nuclei of the hypothalamus known to regulate energy expenditure, and functional studies reveal a role for PACAP in the central regulation of thermogenesis, although mechanisms are not well understood. We hypothesized that PACAP acts upstream of the melanocortin system to regulate sympathetic nerve activity to stimulate thermogenesis. To assess this, female PACAP-/- and PACAP+/+ mice were given daily peripheral injections of a melanocortin receptor agonist, melanotan II (MTII), for 3weeks during cold acclimation, and the effect of MTII on thermogenic capacity and adipose tissue remodelling was examined by physiological and histological analyses. MTII partially rescued the impaired thermogenic capacity in PACAP-/- mice as compared to PACAP+/+ mice as determined by measuring noradrenaline-induced metabolic rate. In addition, MTII treatment during cold acclimation corrected the previously identified deficit in lipid utilization in response to adrenergic stimulation in PACAP-/- null mice, suggesting impaired lipid mobilization may contribute to the impaired thermogenic capacity of PACAP-/- mice. Results presented here provide physiological evidence to suggest that PACAP acts upstream of melanocortin receptors to facilitate sympathetically induced mechanisms of adaptive thermogenesis in response to cold acclimation.

Development of Ligand-Drug Conjugates Targeting Melanoma through the Overexpressed Melanocortin 1 Receptor.

Melanoma is a lethal form of skin cancer. Despite recent breakthroughs of BRAF-V600E and PD-1 inhibitors showing remarkable clinical responses, melanoma can eventually survive these targeted therapies and become resistant. To solve the drug resistance issue, we designed and synthesized ligand-drug conjugates that couple cytotoxic drugs, which have a low cancer resistance issue, with the melanocortin 1 receptor (MC1R) agonist melanotan-II (MT-II), which provides specificity to MC1R-overexpressing melanoma. The drug-MT-II conjugates maintain strong binding interactions to MC1R and induce selective drug delivery to A375 melanoma cells through its MT-II moiety in vitro. Furthermore, using camptothecin as the cytotoxic drug, camptothecin-MT-II (compound 1) can effectively inhibit A375 melanoma cell growth with an IC50 of 16 nM. By providing selectivity to melanoma cells through its MT-II moiety, this approach of drug-MT-II conjugates enables us to have many more options for cytotoxic drug selection, which can be the key to solving the cancer resistant problem for melanoma.

Melanocortin regulation of histaminergic neurons via perifornical lateral hypothalamic melanocortin 4 receptors

ObjectiveHistaminergic neurons of the tuberomammillary nucleus (TMN) are wake-promoting and contribute to the regulation of energy homeostasis. Evidence indicates that melanocortin 4 receptors (MC4R) are expressed within the TMN. However, whether the melanocortin system influences the activity and function of TMN neurons expressing histidine decarboxylase (HDC), the enzyme required for histamine synthesis, remains undefined. MethodsWe utilized Hdc-Cre mice in combination with whole-cell patch-clamp electrophysiology and in vivo chemogenetic techniques to determine whether HDC neurons receive metabolically relevant information via the melanocortin system. ResultsWe found that subsets of HDC-expressing neurons were excited by melanotan II (MTII), a non-selective melanocortin receptor agonist. Use of melanocortin receptor selective agonists (THIQ, [D-Trp8]-γ-MSH) and inhibitors of synaptic transmission (TTX, CNQX, AP5) indicated that the effect was mediated specifically by MC4Rs and involved a glutamatergic dependent presynaptic mechanism. MTII enhanced evoked excitatory post-synaptic currents (EPSCs) originating from electrical stimulation of the perifornical lateral hypothalamic area (PeFLH), supportive of melanocortin effects on the glutamatergic PeFLH projection to the TMN. Finally, in vivo chemogenetic inhibition of HDC neurons strikingly enhanced the anorexigenic effects of intracerebroventricular administration of MTII, suggesting that MC4R activation of histaminergic neurons may restrain the anorexigenic effects of melanocortin system activation. ConclusionsThese experiments identify a functional interaction between the melanocortin and histaminergic systems and suggest that HDC neurons act naturally to restrain the anorexigenic effect of melanocortin system activation. These findings may have implications for the control of arousal and metabolic homeostasis, especially in the context of obesity, in which both processes are subjected to alterations.

Sites and sources of sympathoexcitation in obese male rats: role of brain insulin.

In males, obesity increases sympathetic nerve activity (SNA), but the mechanisms are unclear. Here, we investigate insulin, via an action in the arcuate nucleus (ArcN), and downstream neuropathways, including melanocortin receptor 3/4 (MC3/4R) in the hypothalamic paraventricular nucleus (PVN) and dorsal medial hypothalamus (DMH). We studied conscious and α-chloralose-anesthetized Sprague-Dawley rats fed a high-fat diet, which causes obesity prone (OP) rats to accrue excess fat and obesity-resistant (OR) rats to maintain fat content, similar to rats fed a standard control (CON) diet. Nonspecific blockade of the ArcN with muscimol and specific blockade of ArcN insulin receptors (InsR) decreased lumbar SNA (LSNA), heart rate (HR), and mean arterial pressure (MAP) in OP, but not OR or CON, rats, indicating that insulin supports LSNA in obese males. In conscious rats, intracerebroventricular infusion of insulin increased MAP only in OP rats and also improved HR baroreflex function from subnormal to supranormal. The brain sensitization to insulin may elucidate how insulin can drive central SNA pathways when transport of insulin across the blood-brain barrier may be impaired. Blockade of PVN, but not DMH, MC3/4R with SHU9119 decreased LSNA, HR, and, MAP in OP, but not OR or CON, rats. Interestingly, nanoinjection of the MC3/4R agonist melanotan II (MTII) into the PVN increased LSNA only in OP rats, similar to PVN MTII-induced increases in LSNA in CON rats after blockade of sympathoinhibitory neuropeptide Y Y1 receptors. ArcN InsR expression was not increased in OP rats. Collectively, these data indicate that obesity increases SNA, in part via increased InsR signaling and downstream PVN MC3/4R.

Topical MTII Therapy Suppresses Melanoma Through PTEN Upregulation and Cyclooxygenase II Inhibition.

Melanotan II (MTII), a synthetic analogue of the alpha-melanocyte stimulating hormone (α-MSH), has been applied for skin tanning in humans. However, the carcinogenic consequence of topical MTII has been equivocal. This study aims to delineate the anti-neoplastic efficacy and mechanism of MTII using the B16-F10 melanoma model in vitro and in vivo. It was found that, despite a lack of influence on proliferation, MTII potently inhibited the migration, invasion, and colony-forming capability of melanoma cells. Moreover, topical MTII application significantly attenuated the tumor progression in mice bearing established melanoma. Histological analysis revealed that MTII therapy induced apoptosis while inhibiting the proliferation and neovaluarization in melanoma tissues. By immunoblot and immunohistochemical analysis, it was found that MTII dose-dependently increased the phosphatase and tensin homolog (PTEN) protein level while reducing PTEN phosphorylation, which resulted in the inhibition of AKT/nuclear factor kappa B (NFκB) signaling. Consistently, MTII treatment inhibited cyclooxygenase II (COX-2) expression and prostaglandin E2 (PGE2) production in melanoma cells. Finally, studies of antibody neutralization suggest that the melanocortin 1 receptor (MC1R) plays a critical role in MTII-induced PTEN upregulation and melanoma suppression. Together, these results indicate that MTII elicits PTEN upregulation via MC1R, thereby suppressing melanoma progression through downregulating COX-2/PGE2 signaling. Hence, topical MTII therapy may facilitate a novel therapeutic strategy against melanoma.

Melanotan-II reverses autistic features in a maternal immune activation mouse model of autism.

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder characterized by impaired social interactions, difficulty with communication, and repetitive behavior patterns. In humans affected by ASD, there is a male pre-disposition towards the condition with a male to female ratio of 4:1. In part due to the complex etiology of ASD including genetic and environmental interplay, there are currently no available medical therapies to improve the social deficits of ASD. Studies in rodent models and humans have shown promising therapeutic effects of oxytocin in modulating social adaptation. One pharmacological approach to stimulating oxytocinergic activity is the melanocortin receptor 4 agonist Melanotan-II (MT-II). Notably the effects of oxytocin on environmental rodent autism models has not been investigated to date. We used a maternal immune activation (MIA) mouse model of autism to assess the therapeutic potential of MT-II on autism-like features in adult male mice. The male MIA mice exhibited autism-like features including impaired social behavioral metrics, diminished vocal communication, and increased repetitive behaviors. Continuous administration of MT-II to male MIA mice over a seven-day course resulted in rescue of social behavioral metrics. Normal background C57 male mice treated with MT-II showed no significant alteration in social behavioral metrics. Additionally, there was no change in anxiety-like or repetitive behaviors following MT-II treatment of normal C57 mice, though there was significant weight loss following subacute treatment. These data demonstrate MT-II as an effective agent for improving autism-like behavioral deficits in the adult male MIA mouse model of autism.

Glucose-regulated protein 78 binds to and regulates the melanocortin-4 receptor

The melanocortin-4 receptor (MC4R) belongs to the G protein-coupled receptor (GPCR) family and plays an essential role in the control of energy homeostasis. Here, we identified a novel MC4R-interacting protein, glucose-regulated protein 78 (GRP78), from a pulldown assay using hypothalamic protein extracts and the third intracellular loop of MC4R. We found that MC4R interacted with GRP78 in both the cytosol and at the cell surface and that this interaction increased when MC4R was internalized in the presence of the agonist melanotan-II (MTII). Downregulation of GRP78 using a short interfering RNA approach attenuated MTII-mediated receptor internalization. Reduction in GRP78 expression during tunicamycin-induced endoplasmic reticulum stress also suppressed MTII-mediated internalization of MC4R and cAMP-mediated transcriptional activity. Furthermore, lentiviral-mediated short hairpin RNA knockdown of endogenous GRP78 in the paraventricular nucleus (PVN) of the hypothalamus resulted in an increase in body weight in mice fed a high-fat diet. These results suggest that GRP78 in the PVN binds to MC4R and may have a chaperone-like role in the regulation of MC4R trafficking and signaling.

The stimulatory G protein Gsα is required in melanocortin 4 receptor–expressing cells for normal energy balance, thermogenesis, and glucose metabolism

Central melanocortin 4 receptors (MC4Rs) stimulate energy expenditure and inhibit food intake. MC4Rs activate the G protein Gsα, but whether Gsα mediates all MC4R actions has not been established. Individuals with Albright hereditary osteodystrophy (AHO), who have heterozygous Gsα-inactivating mutations, only develop obesity when the Gsα mutation is present on the maternal allele because of tissue-specific genomic imprinting. Furthermore, evidence in mice implicates Gsα imprinting within the central nervous system (CNS) in this disorder. In this study, we examined the effects of Gsα in MC4R-expressing cells on metabolic regulation. Mice with homozygous Gsα deficiency in MC4R-expressing cells (MC4RGsKO) developed significant obesity with increased food intake and decreased energy expenditure, along with impaired insulin sensitivity and cold-induced thermogenesis. Moreover, the ability of the MC4R agonist melanotan-II (MTII) to stimulate energy expenditure and to inhibit food intake was impaired in MC4RGsKO mice. MTII failed to stimulate the secretion of the anorexigenic hormone peptide YY (PYY) from enteroendocrine L cells, a physiological response mediated by MC4R-Gsα signaling, even though baseline PYY levels were elevated in these mice. In Gsα heterozygotes, mild obesity and reduced energy expenditure were present only in mice with a Gsα deletion on the maternal allele in MC4R-expressing cells, whereas food intake was unaffected. These results demonstrate that Gsα signaling in MC4R-expressing cells is required for controlling energy balance, thermogenesis, and peripheral glucose metabolism. They further indicate that Gsα imprinting in MC4R-expressing cells contributes to obesity in Gsα knockout mice and probably in individuals with Albright hereditary osteodystrophy as well.

Melanotan II causes hypothermia in mice by activation of mast cells and stimulation of histamine 1 receptors.

Intraperitoneal administration of the melanocortin agonist melanotan II (MTII) to mice causes a profound, transient hypometabolism/hypothermia. It is preserved in mice lacking any one of melanocortin receptors 1, 3, 4, or 5, suggesting a mechanism independent of the canonical melanocortin receptors. Here we show that MTII-induced hypothermia was abolished in KitW-sh/W-sh mice, which lack mast cells, demonstrating that mast cells are required. MRGPRB2 is a receptor that detects many cationic molecules and activates mast cells in an antigen-independent manner. In vitro, MTII stimulated mast cells by both MRGPRB2-dependent and -independent mechanisms, and MTII-induced hypothermia was intact in MRGPRB2-null mice. Confirming that MTII activated mast cells, MTII treatment increased plasma histamine levels in both wild-type and MRGPRB2-null, but not in KitW-sh/W-sh, mice. The released histamine produced hypothermia via histamine H1 receptors because either a selective antagonist, pyrilamine, or ablation of H1 receptors greatly diminished the hypothermia. Other drugs, including compound 48/80, a commonly used mast cell activator, also produced hypothermia by both mast cell-dependent and -independent mechanisms. These results suggest that mast cell activation should be considered when investigating the mechanism of drug-induced hypothermia in mice.

Inherently Lean Rats Have Enhanced Activity and Skeletal Muscle Response to Central Melanocortin Receptors.

ObjectiveActivity thermogenesis and energy expenditure (EE) are elevated in intrinsically lean rats (high-capacity runners, HCR), and are also stimulated by melanocortin receptor activation in the ventromedial hypothalamus (VMH). Here, we determined if HCR are more responsive to central modulation of activity EE compared to low-capacity runners (LCR).MethodsHCR and LCR rats received intra-VMH microinjections of Melanotan II (MTII), a mixed melanocortin receptor agonist. Changes in EE, respiratory exchange ratio (RER), activity EE, muscle heat, norepinephrine turnover (NETO), and muscle energetic modulators were compared.ResultsHCR were significantly more responsive to intra-VMH MTII-induced changes in EE, activity EE, NETO to some muscle subgroups, and muscle mRNA expression of some energetic modulators. Though HCR had high muscle activity thermogenesis, limited MTII-induced modulation of muscle thermogenesis during activity was seen in LCR only.ConclusionsAn inherently lean, high-capacity rat phenotype showed elevated response to central melanocortin stimulation of activity EE and use of fat as fuel. This may be driven by sympathetic outflow to skeletal muscle, which was elevated after MTII. Central melanocortin receptor activation also altered skeletal muscle energetic modulators in a manner consistent with elevated EE and lowered RER.

Activation of the central melanocortin system in rats persistently reduces body and fat mass independently of caloric reduction.

Recent evidence indicate that melanotan II (MTII) reduces body mass independently of caloric reduction. Because MTII induces a transient hypophagia, caloric reduction is still considered a primary mechanism for MTII-mediated body mass loss. To examine the contribution of caloric reduction to long-term body mass loss in response to MTII, we centrally infused MTII or vehicle in ad libitum fed (MTII and Control) animals in comparison with a group of animals that were pair-fed (PF) to the MTII group. Food intake and body mass were recorded daily, and body composition was assessed biweekly. The present study demonstrates that central MTII-mediated body mass loss is only partially mediated by caloric restriction, and the long-term body mass loss is independent of the initial hypophagia. More importantly, central MTII administration induced a rapid but sustained fat mass loss, independently of caloric reduction. MTII-treated animals preserved their lean/fat mass ratio throughout the study, whereas PF animals underwent a transient reduction of lean/fat mass ratio that was only normalized when food intake returned to Control level. In summary, it can be concluded that activation of the central melanocortin system in rats persistently reduces body and fat mass independently of caloric reduction.

Effect of Melanotan‐II on Brain Fos Immunoreactivity and Oxytocin Neuronal Activity and Secretion in Rats

Melanocortins stimulate the central oxytocin systems that are involved in regulating social behaviours. Alterations in central oxytocin have been linked to neurological disorders such as autism, and melanocortins have been proposed for therapeutic treatment. In the present study, we investigated how systemic administration of melanotan-II (MT-II), a melanocortin agonist, affects oxytocin neuronal activity and secretion in rats. The results obtained show that i.v., but not intranasal, administration of MT-II markedly induced Fos expression in magnocellular neurones of the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus, and this response was attenuated by prior i.c.v. administration of the melanocortin antagonist, SHU-9119. Electrophysiological recordings from identified magnocellular neurones of the SON showed that i.v. administration of MT-II increased the firing rate in oxytocin neurones but did not trigger somatodendritic oxytocin release within the SON as measured by microdialysis. Our data suggest that, after i.v., but not intranasal, administration of MT-II, the activity of magnocellular neurones of the SON is increased. Because previous studies showed that SON oxytocin neurones are inhibited in response to direct application of melanocortin agonists, the actions of i.v. MT-II are likely to be mediated at least partly indirectly, possibly by activation of inputs from the caudal brainstem, where MT-II also increased Fos expression.